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Study reveals cyclists hit endurance limit in multi-day races such as the Tour de France

Peter Stuart
6 Jun 2019

A study of marathon runners reveals a ‘hard limit’ on endurance that sheds light on cyclists

A study of US ultramarathon runners competing in the Race Across the USA has revealed a ‘hard limit’ on endurance, with wide applications to cycling and specifically events such as the Tour de France.

The study revealed that while athletes like elite cyclists can expend a huge amount of energy on individual days, there’s a ceiling to the energy output that will mean that an athlete's output will flatten out over time.

Specifically, the study claimed that the maximum energy expenditure of an athlete will flatten out at 2.5x their basal metabolic rate. The basal metabolic rate is the amount of energy in calories that someone uses per day while at rest.

For instance, if an individual uses 2,000 calories per day at rest, their maximum energy expenditure will be 5,000 calories over a long period of time. Beyond that the body begins to burn calories quicker than it can absorb them from food. That will result in dipping into the fat reserves, which at a certain point will become exhausted.

The researchers put a rough figure on the point at which an athlete’s maximum energy expenditure would plateau at 20 days. A significant figure for the 23-day Tour de France (21 stages and competitors will ride on the two rest days).

Cycling nutrition

The study considered the energy output of Tour de France cyclists, and found that over the 23-day event athletes managed to produce around 5x their BMR (basal metabolic rate - calories used in a day when at rest), without losing significant weight.

The ability to maintain this higher average, the study believed, could have been down to either nutrition strategies or individual biological deviation. Nevertheless the curve of energy expenditure did level out, though somewhat higher than for average athletes.

To put that in context, though, the maximum energy expenditure of an athlete on a single day of exercise was found to be as hihgh as 9.4x BMR for an 11-hour ultramarathon.

For pro cyclists this may come as no surprise, as drop-outs from the Tour de France alongside drops in maximum heartrate and pure anecdotal wear on the body show, the Tour begins to stretch the physical capacity of many of its riders.

Running studies

A group of academics from several universities, including Purdue University in Indiana and Hunter College in New York, published the study in Science Advances journal. One of the academics, Bryce Carlson, who organised the 4957km Race Across the USA in 2015, saw the opportunity to use the race as a testing ground for a study into endurance capacity.

The runners ran the equivalent of 117 consecutive marathons, and the event lasted 20 weeks for the six sample competitors during which time they were closely monitored to determine their energy expenditure. However, they used a combination of estimates and high quality data to model the metabolic expenditure of cyclists at the Tour de France.

The academics believed that the cause of this endurance ceiling may be the digestive system itself, relating to evolutionary influences which may have limited energy supply.

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